The Hyperporphyrin Concept: A Contemporary Perspective

The Gouterman four-orbital model conceptualizes porphyrin UV–visible spectra as dominated by four frontier molecular orbitals—two nearly degenerate HOMOs and two exactly degenerate LUMOS under D4h symmetry. These are well separated from all the other molecular orbitals, and normal spectra involve transitions among these MOs. Unusual spectra occur when additional orbitals appear in this energy range, typically as a consequence of the central coordinated atom. For example, metals with empty d orbitals in a suitable energy range may lead to charge transfer from porphyrin (ligand) to metal, that is, so-called LMCT transitions. Metals with filled p or d orbitals may lead to charge transfer from metal to porphyrin, MLCT transitions. These cases lead to additional peaks and/or significant redshifts in the spectra and were classified as hyperporphyrins by Gouterman. Cases in which spectra are blueshifted were classified as hypsoporphyrins; they are common for relatively electronegative late transition metal porphyrins. Many of the same principles apply to porphyrin analogues, especially corroles. In this Perspective, we focus on two newer classes of hyperporphyrins: one reflecting substituent effects in protonated or deprotonated free-base tetraphenyporphyrins and the other reflecting “noninnocent” interactions between central metal ions and corroles. Hyperporphyrin effects on spectra can be dramatic, yet they can be generated by relatively simple changes and subtle structural variations, such as acid–base reactions or the selection of a central metal ion. These concepts suggest strategies for engineering porphyrin or porphyrinoid dyes for specific applications, especially those requiring far-red or near-infrared absorption or emission.


INTRODUCTION
Porphyrins and their analogues exhibit a wide range of properties that underlie a myriad of biological roles and an ever-expanding number of applications in chemistry, medicine, and the technological sphere. These have been documented in numerous review articles and the ongoing book series Handbook of Porphyrin Science, now running to 45 volumes with 217 chapters, 1 as well as in numerous review articles (one of the most notable being a special issue of Chemical Reviews on expanded, contracted, and isomeric porphyrins 2 ). A distinctive hallmark of porphyrin-type macrocycles is their rich array of optical properties, which are also reflected in a correspondingly wide range of electronic properties. Furthermore, simple substituentmediated tuning of optical and electronic properties allows for facile application in chemical catalysis, photocatalysis, medicine, and increasingly, materials science and technology. This Perspective aims to summarize and further elucidate the mechanisms underlying the tunability of the optical and electronic properties of porphyrins, specifically the generation and characterization of hyperporphyrins. Martin Gouterman, 3 who passed away recently, is widely regarded as the father of modern porphyrin spectroscopy and introduced the term hyperporphyrin. He is arguably best remembered for his eponymous four-orbital model, which was developed in the early 1960s on the basis of extended Huckel calculations. 4,5 The calculations on the porphyrin π system identified two HOMOs and two LUMOs energetically well separated from all other π molecular orbitals: the two HOMOs are nearly degenerate (a 1u and a 2u under D 4h symmetry), and the two LUMOs are exactly degenerate (e g under D 4h ). Configuration interaction was taken into account, and the characteristic porphyrin Q and B (Soret) bands were accurately assigned to transitions among these four molecular orbitals (Figure 1). Simple perturbation theory arguments then explained why blood (i.e., hemoglobin) is red and grass (i.e., chlorophyll) is green.
Subsequently, Gouterman embarked on a systematic survey of porphyrins and structural analogues, in which he made good use of new compounds synthesized by Buchler, Dolphin, and Adler, among others. In 1978, he presented a masterful optical taxonomy of porphyrins on the basis of their absorption and emission properties. 6 He distinguished three broad classes of porphyrins: normal, hypso, and hyper. These terms are used to describe both the porphyrins and their spectra.
Normal porphyrin spectra refer to those observed for freebase and closed-shell metal (e.g., Mg and Zn) derivatives of simple porphyrins such as tetraphenyl-or octaethylporphyrin. These spectra show the classic Q and B bands, as well as an N band in the near-UV, and are generally well described by the four-orbital model. Early in the development of corrole chemistry, it was established that simple closed-shell metallocorroles, such as axially coordinated Al 7 and Ga 8,9 corroles, also conform to the four-orbital model 10 and exhibit so-called "normal" spectra.
Hypsoporphyrin spectra are similar to normal spectra but with blueshifted Soret and Q bands, exemplified by late transition metal porphyrins involving such elements as Co, Ni, Pd, and Cu. The blueshifts were long thought to reflect backbondinginduced elevation of the porphyrin e g LUMOs. 6 A recent reinvestigation, however, ascribes the hypso effect to lower a 2u HOMO levels in metalloporphyrins with less electropositive metal centers. 11 In contrast, hyperporphyrin spectra show bands that are redshifted relative to normal spectra and, in particular, are defined as showing "prominent extra absorption bands in addition to Q, B, and N in the region λ > 320 nm." 6 Gouterman went on to distinguish between two major classes of hyperporphyrins, the p-type and the d-type: "(1) p-Type hyperporphyrins are found with main group metals in lower oxidation states, that is, Sn(II), Pb(II), As(III), Sb(III), and Bi (III). The extra bands are fairly well established as due to charge transfer (CT) transitions a 2u (np z ) (metal) → e g (π*) (ring). " (2) d-Type hyperporphyrins are found with transition metals in configurations d m , 1 < m < 6, that have holes in the e g (d π ) orbitals and relatively stable lower oxidation states. The extra bands, with somewhat less certainty, are attributed to CT transitions a lu (π), a 2u (π) (ring) → e g (d π ) (metal)." 6 In general, the unique features of metalloporphyrin hyper spectra reflect the presence of additional orbitals in the vicinity of the four porphyrin frontier orbitals. If a central metal (or element) possesses filled valence p orbitals of appropriate symmetry, a metal-to-ligand charge transfer (MLCT) transition is typically observed. However, if the metal harbors empty d orbitals of appropriate symmetry, a ligand-to-metal charge transfer (LMCT) transition is often observed. In both cases, the new transition will necessarily be of lower energy (redshifted) compared with what is predicted by the four-orbital model. The observation and interpretation of hyper spectra led to the correct identification of a number of key heme protein intermediates, especially for thiolate-ligated heme proteins such as chloroperoxidase and cytochrome P450. 12 The present Perspective is not intended to be a comprehensive survey of hyperporphyrins. Instead, after presenting a short introduction to p-and d-type hyperporphyrins, we will focus on two avenues of research on hyperporphyrin systems that we have pursued in recent years in our own laboratories. The first of these, originating largely from the Wamser laboratory, centers around protonated free-base meso-tetraarylporphyrins, for which the redshifted spectral features are thought to reflect aryl-to-porphyrin, i.e., ligand-toligand, charge transfer (LLCT) transitions. The second line of work, originating largely from the Ghosh laboratory, focuses on certain meso-triarylcorrole derivatives, in which similar aryl-tocorrole LLCT transitions account for strong substituent effects on the position of the Soret maxima.

CLASSICAL p-AND d-TYPE HYPERPORPHYRINS
As mentioned above, p-type hyperporphyrin systems typically involve lower-valent p-block element centers with a lone pair. Classic p-type hyperporphyrins include trivalent Group 13 (As, Sb, and Bi) and divalent Group 14 (Ge, Sn, and Pb) porphyrins. 13 Although the first examples were synthesized almost a century ago, some of the classic synthetic work was reported by Johann Buchler, 14 who provided samples to Gouterman for detailed spectroscopic studies. The unsung hero of the field was Phil Sayer, who was a doctoral student of Gouterman and Rex Robinson, subsequently a postdoctoral associate in Gouterman's laboratory, and a meticulous physical chemist. Sayer's experimental work�during the 1970s until his untimely death in 1985, while still a member of Gouterman's laboratory�still forms much of what we know of p-type hyperporphyrin spectra. 13 Figure 2 depicts the classic p-type hyperporphyrin spectra of divalent Ge 15 and Sn 16,17 tetraphenylporphyrin (TPP) derivatives. These are characterized by a split, or shouldered, Soret band and a lowest-energy Q band that extends well into the red or near-infrared. As mentioned, the extra features in these spectra are thought to involve excitations from one or more MOs with p-element lone pair character into the porphyrin LUMOs. Detailed, modern quantum chemical studies of these spectra, however, are yet to be reported. Interestingly, trivalent antimony corroles may provide a fascinating example of a p-type "hypercorrole" spectrum; oxidation to an SbO corrole results in a spectral blueshift, that is, a so-called "normal" corrole spectrum ( Figure 3). 18−20 A contemporary account of p-type hyperporphyrins would be incomplete without some mention of the remarkable axial ligand reactivity of germanium(II) porphyrins. Vaid et al. found that the addition of axial ligands to these complexes results in dramatic electronic-structural and spectral changes, reflecting an intramolecular redox process that effects a two-electron oxidation of the Ge and a two-electron reduction of the porphyrin to isophlorin. 15 Subsequently, the authors reported a similar Si isophlorin system, 21 while Brothers et al. reported an analogous diboron isophlorin. 22 It should be noted that these systems require a careful distinction between valence and oxidation state (the latter being indicated with Roman numerals in Chart 1). 23,24 The archetypal examples of hyperporphyrins�specifically dtype hyperporphyrins�involve middle transition metal deriv-    atives with d electron counts less than six, such as Cr(III), Mn(III), and Fe(III) porphyrins. 6 In contrast, d electron counts of six or higher often lead to hypsoporphyrins. Archetypal examples of the latter include Co(III), Co(II), Ni(II), Pd(II), Pt(II), and Cu(II); simple porphyrin derivatives of these metal ions typically exhibit blueshifted Soret and Q bands relative to their Mg and Zn analogues. Gouterman considered the hyper/ hypso distinction to be significant because, unlike d-type hyperporphyrins, many (but not all) hypsoporphyrins exhibit characteristic emission properties. 6 For example, Pd(II), Pt(II), and Ir(III) porphyrin derivatives are typically strongly phosphorescent. Corrole analogues of hypsoporphyrins include many 5d metallocorroles such as d 2 Os(VI)N corroles, d 6 Ir(III) and Pt(IV) corroles, and d 8 Au(III) corroles, which exhibit both hypsochromically shifted Soret and Q bands as well as near-IR phosphorescence at room temperature. 25 Iron porphyrins exemplify some of the most instructive and important examples of hyperporphyrins. We hasten to add, however, that most six-coordinate hemes as well as their Ru(II) and Os(II) analogues exhibit hypso spectra. 6 A classic example of an iron(II) porphyrin with a hyper spectrum is provided by CO-ligated cytochrome P450. Besides a moderately redshifted Soret band at 446 nm, the enzyme was also found to exhibit a strong near-UV band at 363 nm. Interestingly, such a spectrum could also be generated by passing CO into a solution of ferroprotoheme IX and a thiolate (but not a thiol). In a polarized single-crystal UV−vis study, 26 Hanson et al. (Gouterman's group) established that both bands represented the full concentration of the enzyme (i.e., a single species) and also had the same polarization ( Figure 4). The logical conclusion was made that a near-UV excitation of E u symmetry was mixing heavily with a classic Soret transition to steal the latter's intensity and push it to the red. Subsequent extended Huckel calculations clearly implicated the axial thiolate in determining the Soret and post-Soret absorption profile of P450, with a substantial amount of sulfur character mixing in with the porphyrin a 2u HOMO. 26    The extra 363 nm Soret feature qualifies P450 as a hyperporphyrin, and Hanson and co-workers correctly noted an analogy with p-type hyperporphyrins, which also exhibit split Soret bands. Perhaps most notably, these studies established the heme−thiolate core of cytochrome P450. Much more recently, it has become clear that {FeNO} 6 -heme−thiolate complexes isoelectronic to cytochrome P450 also exhibit qualitatively similar split Soret features ( Figure 5). 27 Higher-valent heme protein intermediates also exhibit hyperporphyrin spectra, as illustrated by the following examples. Figure 6 presents crystal absorption spectra of three different states of the heme−thiolate protein chloroperoxidase (from the fungus Caldariomyces f umago), 28 which catalyzes hydrogenperoxide-mediated halogenation reactions. The spectra depicted are those for the Fe(III)−thiolate resting state, the ferric hydroperoxide state (Compound 0), and a ferric superoxide or oxyheme state (Compound III). Figure 7 depicts the electronic absorption spectra of Compound I and Compound II, which are high-valent Fe(IV) states of the P450 enzyme CYP119. 29−32 Both states exhibit distinctive split Soret bands and weak absorptions in the red/ near-infrared, as expected for hyperporphyrin spectra. It should be noted that the S = 1/2 Compound I state is thought to involve an Fe(IV) center and an antiferromagnetically coupled radical that is delocalized over both the porphyrin and the thiolate axial ligand. The S = 1 Compound II state for this enzyme is believed to involve an Fe IV OH rather than Fe IV O center, which reflects the enhanced basicity of the latter group as a result of the thiolate ligand. Figure 8 depicts the UV−vis spectrum of the Compound II intermediate of catalase (HPC-II), a tyrosinate-ligated heme enzyme from Helicobacter pylori. 33 Unsurprisingly, the spectrum of this Fe IV OH species is qualitatively very similar to that of CPO-II ( Figure 7b). Importantly, the above spectra, and heme protein spectra in general, remain largely unassigned via modern quantum chemical calculations.

PROTONATED meso-TETRAARYLPORPHYRINS AS HYPERPORPHYRINS
Although Gouterman initially described only certain metalloporphyrins (and some metalloid derivatives) as hyperporphyrins, he and his co-workers later also included certain free-base derivatives as hyperporphyrins. 34 A classic example involves the diprotonation of free-base 5,10,15,20-tetrakis(4aminophenyl)porphyrin, H 2 [TAPP]. The diprotonated or "diacid" form, {H 4 [TAPP]} 2+ , exhibits significant bathochromic shifts of both the Q and B bands, as well as an extreme increase in intensity of the far-red Q band ( Figure 9). 35,36 In these cases, the charge-transfer effects obviously cannot be described as MLCT or LMCT but are entirely within the organic ligand; these will be called LLCT transitions. Electronic communication between meso-aryl groups and the porphyrin ring is constrained by their relative spatial orientation. In simple free-base and metal-complexed TPP derivatives, steric interactions between the pyrrole β-hydrogens and the aryl orthohydrogens lead to significant twisting of the aryl groups out of the main porphyrin plane. 37 Thus, aryl substituents typically exert only a modest influence on the electronic character of the porphyrin ring. In general, substituents exert comparable effects on the first oxidation and first reduction potentials, which leads to relatively constant electrochemical HOMO−LUMO gaps (defined as the algebraic difference between the oxidation and reduction potentials) and parallel Hammett plots. 38−40 For a wide range of substituents, the absorption spectra of neutral tetra(p-X-phenyl)porphyrins (TXPPs) are also relatively consistent. 41 With strongly electron-donating substituents (such as alkoxy and amino), however, there is a sharp break in the Hammett plots for oxidation potentials 40 as well as a bathochromic shift of the Q bands, 41,42 which can be interpreted as a gradual impingement of aryl-based MOs into the energy range of the porphyrin's four Gouterman-type frontier MOs. Table 1 presents highlights of substituent effect data on the freebase porphyrins as well as for the diprotonated forms in which strong hyperporphyrin effects appear.
The case of TAPP (the -NH 2 data in Table 1) illustrates that even neutral H 2 [TAPP] may be viewed as an incipient hyperporphyrin, with significant shifts of its Q band and oxidation potential (which serves as an indicator of the relative energy position of the HOMO). Yet, the magnitude of the hyperporphyrin effect induced by protonation is dramatically larger (Figure 9). Diprotonation of the porphyrin ring induces strong nonplanarity (saddling) of the porphyrin ring, primarily as a result of steric repulsion among the four internal pyrrole hydrogens. 43,44 Characteristic hyperporphyrin effects have been observed for a wide range of porphyrin diacids, including those derived from octaethylporphyrin and β-octahalogeno-mesotetraarylporphyrins. 45 The most distinctive cases of free-base hyperporphyrin spectra occur with TPP diacids with strongly electron-donating para-substituents, exemplified by the aforementioned {H 4 [TAPP]} 2+ dication or its dimethylamino analogue. In these cases, protonation occurs preferentially on the central nitrogens even though the peripheral amino substituents are also basic.

Charge Transfer from Multiple Strong Electron Donors to Protonated Porphyrins (Aminophenylporphyrins)
Protonation of various para-aminophenylporphyrins has been the most thoroughly studied, with either amino 35,36 or dimethylamino. 34 as para substituents. Resonance forms for the doubly protonated porphyrin underscore transfer of electronic charge from the amino groups to the porphyrin ring; Chart 2 illustrates representative resonance forms of the three distinct types. We use the categorization proposed by Gouterman and co-workers when they first documented this type of hyperporphyrin. 34 Type A depicts positive charges localized on the porphyrin pyrrole nitrogens; Type B shows one charge delocalized to one of the para-amino groups; and Type C shows both charges delocalized to two para-amino groups, Examination of the resonance forms offers a number of insights that correlate well with experimental observations.
(i) The aryl-porphyrin interactions can sustain up to two charge-transfer interactions of the type described, and cis rather than trans is required to delocalize the two charges. The magnitude of the hyperporphyrin effect can be most readily tracked by the position and extinction coefficient of the far-red Q band. With just two para-amino groups (the others being para-carbomethoxy), the cis regioisomer exhibits a hyperporphyrin band with λ max at 763 nm (ε = 60 mM −1 cm −1 ), while for the trans regioisomer it is at 756 nm (ε = 41 mM −1 cm −1 ). 35 (ii) The hyperporphyrin effect increases with additional donor substituents, where the tri-and tetrasubstituted cases offer multiple modes for the resonance delocalizations of both Type B and Type C. In the limit of four paraamino groups, the hyperporphyrin Q band appears at 813 nm (ε = 89 mM −1 cm −1 ) ( Figure 9). 36,41 (iii) Although monoprotonated porphyrins are rarely observed, 46 they have been proposed for monoamino TPPs where the other three substituents are electron-withdrawing carbomethoxy 35 or sulfonato groups. 47 In these cases, Type B resonance forms are stabilized, but Type C are not, so the second protonation is retarded.  (iv) Excess acid ultimately protonates all the peripheral amino substituents and destroys the hyperporphyrin effect. 34,41 Although the resonance forms are suggestive, the shapes and relative energies of the frontier MOs afford more detailed insight into the origin of the hyperporphyrin effect. The effect has been thought to arise for a single para-aminophenyl group via  elevation of an aminophenyl-based MO to the level of molecular HOMO; in the tetrasubstituted case, both the HOMO and HOMO−1 have been thought to be aminophenyl-based. 48 Thus, the hyperporphyrin transition is described as an aminophenyl-to-porphyrin charge transfer, as suggested by the resonance forms. This picture holds up moderately well, but far from perfectly, in light of modern DFT calculations. NMR, 49,50 FTIR, and resonance Raman 45,51 studies of hyperporphyrin

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Perspective systems support the key structural features implied by the resonance forms, i.e., notably enhanced bonding between the porphyrin meso carbon and the aryl ipso carbon.

Origins of the Hyperporphyrin Effect: Recent TDDFT Results
We have recently reported a TDDFT study aimed at a better understanding of the dramatic hyperporphyrin spectrum of TAPP diacid. 52 Toward this end, we studied both TPP and TAPP (both symmetrized to C 2v ) and their diacids, the latter as their highly symmetric (D 2d ) bisformate complexes (Chart 3). We found it essential to employ both a hybrid functional (such as B3LYP or CAMY-B3LYP) and a solvation scheme (in this case COSMO with CH 2 Cl 2 ) to obtain good simulations of experimental spectra. The results, highlighted here by relevant MO energy level diagrams ( Figure 10) and plots of the relevant MOs ( Figure 11), led to several concrete insights, including multiple factors manifesting themselves as hyperporphyrin spectra. For all four species studied, both free bases and diacids, the Q band consists primarily of HOMO(a 2u )-to-LUMO/LUMO+1 transitions. Two different effects appear to account for the Q band redshifts. For diprotonation of the free-base forms, the major factor underlying Q band redshifts is a lowering of the LUMOs as a result of infusion of meso-aryl character. Elevation of the "a 2u " HOMO plays a smaller role. In contrast, the redshifted Q band of free-base H 2 [TAPP] relative to H 2 [TPP] reflects destabilization of the "a 2u " HOMO because of antibonding "filled−filled" interactions with aminophenylbased occupied MOs, while the LUMOs are less affected energetically.
Beyond the Q bands (i.e., for the Soret bands as well as certain pre-Soret and post-Soret bands), the transitions of the diacid forms are more complex, with meso-aryl → LUMO character mixing in with classic Gouterman "a 1u " → LUMO transitions. Indeed, some of these transitions may be described as primarily meso-aryl or aminophenyl-based.

Charge Transfer from Deprotonated Electron Donors to Neutral Porphyrins (Hydroxyphenylporphyrins)
5,10,15,20-Tetrakis(p-hydroxyphenyl)porphyrin (THPP) and related derivatives have been studied extensively by Milgrom and others for many years because of their unusual property of being very easily oxidized to phenoxy radicals, even in air for some derivatives. 53 This observation already suggests that the HOMO is localized on the hydroxyphenyl group and that spectrophotometric titrations of H 2 [THPP] and analogues must be carried out with the careful exclusion of oxygen. Base titrations of TPPs with one to four para-hydroxy groups show clear hyperporphyrin spectra, 54 assigning the dianionic forms to hyperporphyrins via the resonance structures shown in Chart 4. Resonance forms of Type B are not shown but are also possible. In this case, an anionic, strongly electron-donating group is generated via the deprotonation of the phenol substituents while the porphyrin remains uncharged. Similar hyperporphyrin effects have also been demonstrated with Ni[THPP] in a strong base. 45 In the case of monosubstituted p-hydroxyphenylporphyrin, the choice of solvent can affect whether a hyperporphyrin spectrum is observed. In DMF, deprotonation leads to the

Push−Pull Charge Transfer through Protonated Porphyrins (Aminophenyl/Pyridylporphyrins)
Type C resonance forms (Charts 3 and 4) feature two double bonds that are exocyclic to the porphyrin ring. In the limit of four such exocyclic double bonds, the derivatives are called oxoporphyrinogens, which are generally formed via the oxidation of porphyrins. 56 The most common example of an oxoporphyrinogen is that formed by oxidation of 5,10,15,20tetrakis(3,5-t-butyl-4-hydroxyphenyl)porphyrin, a THPP analogue that incorporates the steric hindrance of t-butyl groups to stabilize the oxidized form. Because the pyrrole nitrogens in these cases are readily derivatized, these oxoporphyrinogens have been studied as catalysts and sensors, as well as in other applications. 57 One case of a hyperporphyrin with a proposed oxoporphyrinogen resonance form has been reported, in which the additional exocyclic double bonds are formed by charge transfer through the porphyrin core between an electron-rich aryl group and an electron-deficient aryl group. Thus, acid titration of 5 , 1 0 , 1 5 -t r i s ( 4 -a m i n o p h e n y l ) -2 0 -p y r i d y l p o r p h y r i n (H 2 [TA 3 PyP]) shows a hyperporphyrin spectrum at the triprotonated state (Figure 12). 36 The diprotonated state is not observed during the titration, as indicated by clean isosbestic points during the titration from +1 to the +3 state. At the triprotonated stage, two aminophenyl groups delocalize the positive charge from the two interior protonations, while the third protonation forms a pyridinium group that can interact with the third aminophenyl group in a push−pull charge transfer across the porphyrin ring (Chart 5). This resonance form with four exocyclic double bonds represents a new type for hyperporphyrins (Type D).

Enhanced Charge Transfer via Ethynyl Linkers (Arylethynylporphyrins)
Aryl-to-porphyrin charge transfer interactions are enhanced when ethynyl linkers are inserted between the porphyrin meso positions and phenyl substituents, allowing for greater coplanarity of the two rings. The cases studied included paradimethylamino 58 and para-hydroxy substituents, 59 but were limited by the synthetic method to two ethynyl-linked aryl groups located trans to one another. For protonation of the porphyrin with trans dimethylaminophenyls and ethynyl linkers, the Q band at 802 nm (ε = 49 mM −1 cm −1 ) was found to be considerably more redshifted (but less intense) than that in the analogue without the ethynyl linkers (723 nm, ε = 60 mM −1 cm −1 ). The corresponding porphyrins with para-hydroxyphenyl groups were studied in both acidic and basic media. In acidic medium, the hyperporphyrin effect of hydroxy groups was found to be smaller but nonetheless enhanced with the ethynyl linkers (736 nm with ethynyl linkers and 671 nm without). In basic medium, the λ max values are 752 nm with ethynyl linkers and 671 nm without.

Hyperporphyrins Based on Charge Transfer from Porphyrin to Aryl Have Not Been Observed
In theory, one can imagine a variety of different circumstances in which charge transfer can be induced between a porphyrin and an attached aryl group, as illustrated in Chart 6.
In this paper, we have cited both types of aryl-to-porphyrin LLCT, specifically protonated TAPP as emblematic of Type I and deprotonated THPP of Type II. To our knowledge, hyperporphyrins in which charge transfer goes from porphyrin to aryl substituents have not yet been reported in the literature. Protonation of pyridylporphyrins (and N-methylpyridiniumyl) do not show hyperporphyrin effects as suggested in Type I′. Deprotonation should make the porphyrin ring a much stronger electron donor as suggested in Type II′. However, deprotonation of tetrakis(4-nitrophenyl)porphyrin (TNPP) was reportedly unsuccessful using TBAOH in various solvents. 60 Tetrakis-(N-methyl-4-pyridiniumyl)porphyrin (TMPyP) has been reported to be deprotonated in aqueous solution with an apparent pK a of 12.6; however, the proposed result was a single deprotonation and an unremarkable spectrum. 61−63 The interior pyrrolic hydrogens of porphyrins are weakly acidic, even with strong electron-withdrawing substituents, and it may be that stronger base systems will be required to observe a hyperporphyrin with a porphyrin-to-aryl charge-transfer.
Beyond the four types of LLCT interactions described in Chart 6, one could imagine a push−pull hyperporphyrin that did not rely upon either protonation or deprotonation. In such a case, the presence of both strong electron donation and electron withdrawal at different sites on a porphyrin ring could lead to a push−pull resonance form analogous to that shown as Type D in Chart 5. A large number of porphyrins, including some with appropriate push−pull substituents, have been prepared for different applications, for example, as anticancer agents, but none showed hyperporphyrin behavior in aqueous solution. 64 A copper porphyrin with trans meso substituents of dimethylaminophenylethynyl and nitrophenylethynyl also showed no hyperporphyrin effects. 65 We are not aware of any free-base porphyrins that show hyperporphyrin effects without the assistance of an acid or base.

Hyperporphyrins in Redox and Photoredox Reactions
Aside from protonation reactions, porphyrins can also acquire positive charges via oxidation. Here, again, strongly electrondonating groups engender hyperporphyrin effects. Hyperporphyrin effects in the oxidation of H 2 [TAPP] have been tracked by oxidative titration or by spectroelectrochemistry. 66,67 For the doubly oxidized species {H 2 [TAPP]} 2+ , both cis and trans resonance forms of Type C′ are possible (Chart 7).
The oxidation of H 2 [TAPP] is complicated by polymerization, analogous to the oxidative polymerization of aniline. Reversible electrochemical oxidation of a poly-TAPP film also shows clear hyperporphyrin spectra at positive potentials. In this case, individual porphyrin units in the polymer can still be described as having characteristic exocyclic Type C′ resonance forms; in fact, these structures are considered critical to the electronic conductivity observed for poly-TAPP films. 66,67 The polymerization of TAPP is minimized in acidic media, and titration of the monomeric, fully protonated species {H 8 [TAPP]} 6+ with ammonium persulfate in aqueous acid results in typical hyperporphyrin spectra upon oxidation. 67 In acidic media, protonated amino substituents would ordinarily fail to yield hyperporphyrin effects. In this case, however, oxidation followed by the loss of two protons generates a +6 form that can still sustain hyperporphyrin resonance, i.e., Type C′ forms in which additionally both pyrroles and both anilino groups are protonated.

Free-Base Hypercorroles
The Gouterman four-orbital model has been successfully applied to corroles; the four frontier orbitals of unsubstituted gold corrole, Au[Cor], are depicted in Figure 13. 10,68 The spectra of free-base corroles present a number of interpretational challenges. The molecules are not only strongly nonplanar as a result of steric repulsion among the three central hydrogens, but they also exist as a mixture of two tautomers. Furthermore, freebase corroles are partially to fully ionized in many common solvents (such as DMF and DMSO), even in the absence of an added base. 69−72 Free-base corroles, however, can only undergo a single protonation at their cores, and the protonated site can only interact with three meso substituents. While these features distinguish free-base corroles from porphyrins, many of the same principles of hyperporphyrin spectra apply.
Acid titrations of all the isomers (o,m,p) of meso-tris-(aminophenyl)corrole, H 3 [TAPC], have been studied. 73 Comparison of the para TAPC isomer with the corresponding porphyrin (TAPP) is particularly instructive. Successive protonations ( Figure 14) lead first to neutralization of the anion to form the neutral corrole and subsequently to the monoprotonated form {H 4 [TAPC]} + with clear hyperporphyr-in (hypercorrole) characteristics. Upon treatment with excess acid, the spectrum returns to normal.
Chart 8 depicts the resonance forms of the para isomer of H 4 [TAPC]} + ; one localizes the positive charge on a pyrrole nitrogen (Type A), and there are three options for delocalizing the charge to aminophenyl groups (Type B). Unlike for TAPP diacid (see Chart 2), a Type C resonance form is not possible. The resonance forms suggest that {H 4 [TAPC]} + has fewer pathways for charge delocalization interactions (as well as fewer aminophenyl substituents) relative to TAPP diacid. The net result is that the hyperporphyrin effect in TAPP diacid (see Figure 9) is distinctly stronger than what is observed for {H 4 [TAPC]} + . Thus, the Q band positions and extinction coefficients are 813 nm (89 mM −1 cm −1 ) for TAPP diacid 36 and 764 nm (20 mM −1 cm −1 ) for {H 4 [TAPC]} + . 73 As the peripheral amino groups become successively protonated (Figure 14c), the effects gradually diminish, without clear isosbestic points since multiple species are present, and the spectrum ultimately returns to a normal Q band at 687 nm.

METALLOTRIARYLCORROLES
While many 4d and 5d corroles presumably exhibit hypso spectra (a point that still needs verification), 11 many first-row transition metal corroles clearly exhibit d-type hyper spectra. 25,74 Among the latter, many (but not all) meso-triarylcorrole derivatives exhibit a remarkable substituent effect, which is not observed for meso-tetraarylporphyrins. The Soret maximum in these systems redshifts systematically with increasing electrondonating character of the para substituent on the meso-phenyl groups. Such substituent effects are particularly well-established for Mn, Fe, Co, and Cu corroles ( Table 2). A variety of probes have established that the effect is specific to noninnocent metallocorroles, i.e., those having partial corrole •2− radical character, which typically arises via one of two orbital interactions depicted in Figure 15. 68,74 Although few of these spectra have been theoretically analyzed, a TDDFT study of copper triarylcorroles suggests that the substituent-sensitive components of the Soret manifolds are aryl-to-corrole •2− charge transfer transitions, 75 not unlike a number of LLCT transitions mentioned above. Some of the main classes of noninnocent metallocorroles are described below.

The Manganese Case
While all Mn corroles exhibit complex d-type hyper spectra relative to "normal" Al and Ga corroles, the Mn[TpXPC]Cl series exhibits substituent-sensitive Soret bands, implicating a noninnocent Mn III -corrole •2− description. In contrast, the Mn(III) and Mn[TpXPC]Ph series ( Figure 16) are thought to involve an innocent corrole. 76

The Iron Case
In an exact parallel to Mn triarylcorroles, FeCl 77−79 triarylcorroles exhibit substituent-sensitive Soret maxima clearly indicative of aryl-to-corrole LLCT transitions and hyperporphyrin character, but FePh triarylcorroles do not ( Figure  17). In an unexpected development, FeNO 80 triarylcorroles were found to exhibit substituent-sensitive Soret maxima, suggesting a novel {FeNO} 7 -corrole •2− description, which was later supported by several other lines of evidence. Similarly, μoxo diiron triarylcorroles also exhibit mildly substituentsensitive Soret maxima, suggesting the following intramolecularly spin-coupled description which was also supported by broken-symmetry DFT calculations. 81

Cobalt Corroles
One of our more surprising findings in recent years is that the five-coordinate Co[TpXPC](PPh 3 ) 82 ( Figure 18) and Co-[TpXPC](py) 83 ( Figure 19) series do not involve classic lowspin Co(III) centers but are best described as Co II -corrole •2− . Again, hypercorrole spectra with substituent-sensitive Soret maxima provided the first clue, which was subsequently augmented with several other lines of evidence. In contrast, the six-coordinate Co[TpXPC](py) 2 series gives substituentinsensitive Soret maxima and are best thought of as genuine lowspin Co(III) complexes. 83 In nonpolar solvents such as dichloromethane, however, one of the pyridine ligands falls off and the resulting solutions, in which the main species is Co[TpXPC](py), exhibit substituent-sensitive Soret maxima ( Figure 19). Worth noting in this connection is that the Rh[TpXPC](PPh 3 ) 83 series involves innocent corrole macrocycles.

The Coinage Metals
Among copper corroles, 84 (Figure 20), indicative of a Cu II -corrole •2− description. Gold triarylcorroles, in sharp contrast, do not exhibit such substituent sensitivity, which indicates (on the basis of other additional lines of evidence) an innocent Au III -corrole 3− electronic description. 87−89 This difference manifests itself most dramatically in the structures of isoelectronic coinage metal corroles: while Au corroles are planar, Cu corroles, uniquely among metallocorroles, are inherently saddled. The saddled conformation is associated with a Cu(d x 2 −y 2 )-corrole(π) orbital interaction (depicted in Figure 15), which allows part of the corrole(π) electron density to flow into the formally empty Cu(d x 2 −y 2 ) orbital, resulting in an overall Cu II -corrole •2− description. In the Au case, the relativistically destabilized 5d x 2 −y 2 orbital is too high in energy to engage in a similar interaction, explaining both the substituent-insensitive Soret bands in the Au[TpXPC] series and the planar macrocycle geometries. 88

Platinum Corroles
A series of neutral, paramagnetic complexes Pt[TpXPC](Ar)-(Ar′) exemplify some of the best examples of uncoupled corrole radicals. 90 The complexes, which may be described as Pt IV [TpXPC • ](Ar)(Ar′), exhibit hypercorrole spectra with prominent split Soret bands in which the main visible peak is strongly substituent-dependent ( Figure 21). In contrast, the true Pt(IV) series Pt[TpXPC](Ar)(py) exhibits much sharper, slightly split Soret bands, whose positions are essentially substituent-independent. 91 These probably also have some hyper character, but theoretical assignments are still lacking.

CONCLUSIONS AND PROSPECTS
We have presented a contemporary and somewhat personal perspective of hyperporphyrin spectra by focusing on two major classes of compounds: centrally protonated meso-tetraarylporphyrins and noninnocent metallotriarylcorroles. Classic p-type and d-type hyperporphyrins have also been briefly covered, while a discussion of spectral redshifts due to conjugating βsubstituents (such as halogen, alkoxy, alkylthio, amino, etc.) has been deferred for a different occasion.
Hyperporphyrin spectra can reflect various types of charge transfer transitions�MLCT, LMCT, LLCT, and combinations thereof. The term thus appears to unite a class of related electronic phenomena and a rather diverse menagerie of molecules that exhibit them. Because of the importance of porphyrins as optoelectronic materials, the ability to create hyperporphyrin effects by simple structural perturbations such as the complexation of certain elements, certain types of peripheral substituents, and acid−base reactions may be a particularly useful paradigm. Compared with Gouterman's days, there are many new approaches to the design of NIR-absorbing and emitting dyes, such as porphyrin ring reduction, 92,93 loss of aromaticity, 94,95 and especially porphyrinoids with extended conjugations (e.g., nanographenes). 96 In spite of the plethora of options, the hyperporphyrin paradigm remains attractive on account of its practical simplicity. Below are some musings on how the concept can serve as a creative force in both fundamental and application-oriented areas.
Let us consider hemes and their model compounds, including metallocorroles, that exhibit hyperporphyrin spectra. A deeper understanding of the spectra may improve our understanding of their reactivity and potentially facilitate their deployment as catalysts and therapeutics (e.g., as antioxidants for disorders mediated by oxidative stress). Such an understanding may be further enhanced by MCD 97 and XAS spectroscopy. 78 Many such studies of "electronic structure contributions to reactivity" have been reported by Solomon et al., but only rarely for heme and porphyrin-type systems. 98,99 Such studies would be a welcome addition to the heme and metalloporphyrin literature.
A significant amount of interest in porphyrins centers around their potential application to solar energy conversion, analogous to the role of chlorophyll in photosynthesis. A push−pull charge transfer motif has been at the heart of many synthetic porphyrins designed for this purpose. 100−102 Thus, some of the most efficient photosensitizers employed in dye-sensitized solar cells are porphyrins with an N,N-diarylamino meso substituent and a carboxyphenyl anchoring group. 103,104 The unique Type D resonance form illustrated earlier has been suggested as a potential two-electron hydride donor by analogy to the NADH/

JACS Au
pubs.acs.org/jacsau Perspective NAD + couple ( Figure 22). 36 In this case, the N−H bond of a protonated pyridine is suggested as a hydride donor, leaving behind an oxidized porphyrin. Such a process might be photochemically induced.
In spite of their attractive spectra for solar absorption, typically extending into the far red, hyperporphyrin systems based on protonated tetraphenylporphyrins have not been directly examined as photosensitizers. Presumably, because it is necessary to generate such systems under strongly acidic conditions, they have been little used for solar sensitization, as well as biomedical applications such as in photodynamic therapy. That said, protonated tetraphenylporphyrins have been used as sensors for gases such as ammonia, hydrogen sulfide, and sulfur dioxide. 105,106 As a final example, 5d metalloporphyrins and metallocorroles, which are of great interest as triplet photosensitizers, might be considered. 25,107−109 Gouterman himself devoted the latter part of his career to developing phosphorescent Pt porphyrins as pressure-sensitive paints for airplane wings. 110,111 In one of our own laboratories, we are developing 5d metallocorroles as dyes for oxygen sensing, photodynamic therapy, and triplet−triplet annihilation upconversion. 25 Although several of these complexes are actually of the hypso type, the use of suitable substituents might significantly redshift key absorption and emission features. 11 In such a case, the hypso/hyper distinction might become moot. We hope to illustrate such applications of the hyperporphyrin concept by synthesizing new classes of 5d metallocorrole-based photosensitizers. ■ REFERENCES